To code these spatial information parameters, the bit budget available, depending on the coders, is not always sufficient. In the case of frequency sub-band coding, this budget is divided per sub-band.
There exist techniques which make it possible to reduce the number of bits to be allocated per sub-band. One of these techniques consists in coding only the parameters of one frequency band out of two for each temporal frame. Thus the sub-bands not coded in the current frame are allotted the corresponding values of the previous frame.
Another technique is to perform an intra or inter-frame differential coding.
Most of the time, these allocation techniques are not based on criteria of auditory perception that a listener may have of the sound signal. Therefore, these parameters are quantized in a uniform manner.
A quantization based on psycho-acoustic criteria is proposed by Breebaart in the document by Breebaart, J; Van de Par, S; Kohlrausch, A & Schuijers, E, “Parametric Coding of stereo Audio” in EURASIP Journal on Applied Signal Processing, 2005, 9, pp 1305-1322. The scheme described in this document is based on the perception that a listener may have on certain frequency bands for particular parameters of inter-channel difference type, or on the sensitivity to a variation of these parameters as a function of the relevant span of values. It is for example described that certain parameters are coded only on the frequency bands below 1 kHz. Beyond this frequency, the parameters are indeed no longer useful to the auditory system to locate a source. Thus, the psycho-acoustic criterion used here relates to a sensitivity to the coded parameters and not to a sensitivity of spatial displacements of the sound sources.
Now, auditory perception or sensitivity with respect to a spatial resolution in the sub-bands may vary at each instant from one sub-band to another, independently of the parameter to be coded.